Nerve and Muscle Physiology Practice Questions

Q: Which ion is most crucial for the propagation of action potentials in neurons?

Sodium. ***Sodium*** - The rapid influx of **sodium ions** through voltage-gated sodium channels is responsible for the **depolarization phase** of the action potential. - This depolarization is the primary event that triggers and propagates the nerve impulse along the axon. *Calcium* - While calcium is crucial for **neurotransmitter release** at the synaptic terminal, it is not the primary ion responsible for the initial depolarization and propagation of the action potential in the axon itself. - Influx of **calcium ions** plays a more significant role in action potentials in certain cell types like cardiac muscle cells. *Magnesium* - Magnesium acts as a cofactor for many enzymes and plays a role in nerve conduction indirectly, but it is not directly involved in the rapid ion flux that generates an action potential. - High magnesium levels can actually **reduce neuronal excitability** by blocking NMDA receptors. *Potassium* - **Potassium ions** are primarily responsible for the **repolarization phase** of the action potential, where they exit the cell to restore the negative resting membrane potential. - Their efflux also contributes to the **hyperpolarization** that follows an action potential.

Q: A patient is being evaluated for an electrolyte imbalance. Which electrolyte is most critical for the transmission of nerve impulses?

Sodium. ***Sodium*** - **Sodium ions** play a crucial role in establishing and maintaining the **resting membrane potential** in neurons, and their rapid influx through voltage-gated channels is essential for the **depolarization phase of an action potential**. - The movement of sodium across the neuronal membrane drives the **propagation of nerve impulses** along the axon. *Calcium* - While essential for **neurotransmitter release** at the synaptic terminal, **calcium** is not the primary electrolyte responsible for the creation and propagation of the action potential along the nerve fiber itself. - Its main role in nerve transmission is modulating the exocytosis of vesicles containing neurotransmitters. *Magnesium* - **Magnesium** is an important cofactor for many enzymes and plays a role in stabilizing membranes, but it does not directly participate in the rapid changes in membrane potential that characterize nerve impulse transmission. - It can act as a **calcium channel blocker** and influence neurotransmitter release, but it's not the primary ion for impulse generation. *Potassium* - **Potassium ions** are crucial for the **repolarization phase** of the action potential, moving out of the cell to restore the resting membrane potential. - Although vital for resetting the neuron, potassium's primary role is not in the initial firing or depolarizing phase of the nerve impulse.

Q: A patient exhibits hyperkalemia. Which of the following explains how this condition affects nerve excitability?

Increases resting membrane potential. ***Increases resting membrane potential (makes it less negative/depolarized)*** - **Hyperkalemia** (elevated extracellular K⁺) **depolarizes the resting membrane potential**, making it **less negative** (e.g., from -70 mV to -60 mV). - This occurs because the **Nernst equilibrium potential for K⁺** becomes less negative when extracellular K⁺ increases, shifting the resting potential closer to 0 mV. - The membrane potential moves **closer to the threshold** (which remains constant at ~-55 mV), **initially increasing excitability**. - However, prolonged depolarization causes **inactivation of voltage-gated Na⁺ channels**, leading to **paradoxical decreased excitability**. - Note: The term "increases" here means the membrane potential becomes **less negative** (moves toward 0 mV), not that it becomes more polarized. *Increases action potential threshold* - The **action potential threshold remains constant** at approximately -55 mV and does **not change** with hyperkalemia. - What changes is the **resting membrane potential**, not the threshold. *Decreases action potential threshold* - The **threshold potential does not decrease** in hyperkalemia; it remains fixed at ~-55 mV. - The misconception arises from confusing the **gap between resting potential and threshold** (which decreases) with the **threshold itself** (which stays constant). - While the membrane potential moves closer to threshold, the threshold value itself is unchanged. *Decreases membrane potential* - This phrasing is ambiguous. If "decreases" means becoming **more negative** (hyperpolarization), this is incorrect—hyperkalemia causes **depolarization** (less negative). - If "decreases" means the **absolute value decreases** (e.g., from -70 mV to -60 mV, moving toward 0), this could be correct but is poorly worded. - The preferred terminology is that hyperkalemia **depolarizes** the membrane or makes it **less negative**.

Q: Which type of muscle contraction involves the shortening of the muscle while generating force?

Concentric. ***Concentric*** - **Concentric contraction** occurs when the muscle shortens under tension, overcoming the resistance. - This type of contraction is responsible for moving a load or accelerating a body segment. *Isometric* - **Isometric contraction** involves muscle activation without a change in muscle length; the force generated by the muscle equals the load. - An example is holding a heavy object steady in one position, where no movement occurs. *Eccentric* - **Eccentric contraction** occurs when the muscle lengthens while under tension, often acting as a brake against an external force. - This type of contraction typically happens during the controlled lowering of a weight or decelerating a body segment. *Isokinetic* - **Isokinetic contraction** involves muscle contraction at a constant velocity throughout the range of motion, requiring specialized equipment to maintain this speed. - The force generated by the muscle varies to keep the speed constant, typically seen in rehabilitation settings.

Q: A 30-year-old patient presents with muscle weakness and fatigue. Which ion channel disorder is MOST COMMONLY responsible for these symptoms?

Potassium channel. ***Potassium channel*** - **Potassium channelopathies** often lead to **muscle weakness and fatigue**, as these channels are critical for muscle cell repolarization and excitability. - Examples include **periodic paralyses**, where abnormal potassium channel function causes episodes of severe muscle weakness [2]. *Sodium channel* - **Sodium channelopathies** usually manifest as **painful myotonia** or **paralysis**, rather than general weakness [2]. - Conditions like **hyperkalemic periodic paralysis** involve sodium channel dysfunction but present distinctly with episodes of flaccid paralysis triggered by high potassium levels [2]. *Calcium channel* - **Calcium channel disorders** are more commonly associated with **neurological symptoms** like ataxia, migraines, or specific muscle diseases like **Lambert-Eaton myasthenic syndrome**, which primarily causes proximal muscle weakness [1]. - Familial hemiplegic migraine is another disorder linked to calcium channel dysfunction [3]. *Chloride channel* - **Chloride channelopathies** are known to cause conditions like **myotonia congenita**, characterized by **muscle stiffness and delayed relaxation** following contraction, not typically general muscle weakness and fatigue [2]. - The reduced chloride conductance leads to hyperexcitability of the muscle membrane.

Q: In a muscle fiber at rest, the length of the I band is 1 mm and A band is 1.5 mm. What is the length of the sarcomere?

2.5 mm. ***2.5 mm*** - A **sarcomere** is defined as the region between two successive **Z-discs** and is the functional contractile unit of muscle. - Each sarcomere contains: one complete **A band** (1.5 mm) in the center, and **half of each adjacent I band** on either side. - The given **I band length of 1 mm** represents the full I band, so each sarcomere contains two halves of I bands: 0.5 mm + 0.5 mm = 1 mm total. - Therefore, sarcomere length = **A band + two half I-bands** = 1.5 mm + 0.5 mm + 0.5 mm = **2.5 mm**. *0.5 mm* - This value represents only half of the I band length, which is the portion of I band within one sarcomere from one side. - This does not account for the **A band** or the other half of the **I band**, so it grossly underestimates sarcomere length. *3.5 mm* - This incorrect value would result from adding the **A band** to two complete **I bands** (1.5 mm + 1 mm + 1 mm = 3.5 mm). - This is wrong because each sarcomere contains only **two halves** of I bands (totaling one I band), not two complete I bands. *5 mm* - This value has no anatomical basis in sarcomere structure given the measurements provided. - It represents a significant overestimation and misunderstanding of how the **A band** and **I band** components sum to form the sarcomere length.

Q: Afferents for stretch reflexes are carried by which fibers?

Ia (Group Ia). ***Ia (Group Ia)*** - **Type Ia (Group Ia) afferent fibers** from muscle spindles are the primary sensory receptors that carry information about **muscle stretch and velocity of stretch**. - Type Ia fibers are the **largest diameter** and **fastest conducting sensory fibers** in the peripheral nervous system, ensuring rapid transmission for the monosynaptic stretch reflex. - They synapse directly on alpha motor neurons in the spinal cord, forming the basis of the **stretch reflex (myotatic reflex)**. - **Type II (Group II) afferents** also contribute to stretch reflexes by detecting static muscle length. *Gamma (γ) efferents* - **Gamma motor neurons** are **efferent (motor) fibers**, not afferent (sensory) fibers. - They innervate the **intrafusal muscle fibers** within the muscle spindle to regulate spindle sensitivity. - Their role is motor control of spindle tension, not carrying sensory information from the stretch reflex. *Type B* - **Type B fibers** are **preganglionic autonomic fibers** (sympathetic and parasympathetic). - They are **myelinated** but smaller and slower than Type Ia fibers, and are not involved in somatic stretch reflexes. *Type C* - **Type C fibers** are **unmyelinated**, small-diameter, and the **slowest conducting** nerve fibers. - They primarily transmit **pain (nociception)**, **temperature**, and some **autonomic** signals, not proprioceptive information for stretch reflexes.

Question 1

Which ion is most crucial for the propagation of action potentials in neurons?

Accepted Answer

Sodium

Answer

Calcium

Answer

Magnesium

Answer

Potassium

Question 2

A patient is being evaluated for an electrolyte imbalance. Which electrolyte is most critical for the transmission of nerve impulses?

Accepted Answer

Sodium

Answer

Calcium

Answer

Magnesium

Answer

Potassium

Question 3

A patient exhibits hyperkalemia. Which of the following explains how this condition affects nerve excitability?

Accepted Answer

Increases resting membrane potential

Answer

Increases action potential threshold

Answer

Decreases action potential threshold

Answer

Decreases membrane potential

Question 4

A patient with hyperkalemic periodic paralysis experiences muscle weakness. How does this condition affect muscle excitability?

Accepted Answer

Depolarizes RMP, leading to muscle fiber inexcitability

Answer

Shifts sodium channel inactivation kinetics

Answer

Depolarizes RMP, reducing sodium channel availability

Answer

Hyperpolarizes RMP, impairing action potential generation

Question 5

What role does the sodium-potassium pump play in neuronal activity?

Accepted Answer

It restores resting membrane potential after action potentials

Answer

It generates action potentials

Answer

It directly generates the depolarization phase

Answer

It synthesizes neurotransmitters

Question 6

Which type of muscle contraction involves the shortening of the muscle while generating force?

Accepted Answer

Concentric

Answer

Isometric

Answer

Eccentric

Answer

Isokinetic

Question 7

A 30-year-old patient presents with muscle weakness and fatigue. Which ion channel disorder is MOST COMMONLY responsible for these symptoms?

Accepted Answer

Potassium channel

Answer

Sodium channel

Answer

Calcium channel

Answer

Chloride channel

Question 8

Excitability of cells is maximally affected by a change in the concentration of which ion?

Accepted Answer

Ca2+

Answer

Na+

Answer

K+

Answer

Cl-

Question 9

In a muscle fiber at rest, the length of the I band is 1 mm and A band is 1.5 mm. What is the length of the sarcomere?

Accepted Answer

2.5 mm

Answer

0.5 mm

Answer

3.5 mm

Answer

5 mm

Question 10

Afferents for stretch reflexes are carried by which fibers?

Accepted Answer

Ia (Group Ia)

Answer

Gamma (γ) efferents

Answer

Type B

Answer

Type C

Nerve and Muscle Physiology — MCQs

Nerve and Muscle Physiology — MCQs

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